Gyroscopes are used to measure rotation rates or changes in angular velocity about an axis. A resonant fiber optic gyroscope (RFOG) includes a light source, a beam generating device, and an optical fiber coil coupled to the beam generating device. The beam generating device transmits light beams into the coil that propagate in a clockwise (CW) direction and a counter-clockwise (CCW) direction along the core of the optical fiber. Many RFOGs utilize glass-based optical fibers that conduct light along a solid core of the fiber. The two counter-propagating (CW and CCW) light beams experience different path lengths while propagating around a rotating path, and the difference in the two path lengths is proportional to a rate of rotation around the gyroscope's axis.
In general, the size of the RFOG affects the accuracy or sensitivity of a RFOG. For example, smaller RFOGs typically have less accuracy than larger RFOGs. RFOGs have accuracies that generally increase with the diameter of the light path formed by the optical fiber coil, and traversed by the light beams. Thus, the larger the coil diameter—the greater the signal-to-noise ratio of the RFOG. Also, to improve the signal-to-noise ratio of the RFOG, the optical path may be increased by increasing the number of turns of the optical fiber coil.
In a resonant fiber optic gyro, the optical fiber coil acts as a resonator. The counter-propagating light beams in the optical fiber coil are monochromatic. The counter-propagating light beams circulate through multiple turns of the optical fiber coil, and recirculate multiple times through the optical fiber coil using a recirculating device such as an optical fiber coupler or a beam splitter. The recirculating device also introduces light into and extracts light out of, the optical resonator. The beam generating device typically modulates and/or shifts the frequencies of each of the counter-propagating light beams so that the resonant frequencies of the resonator formed by the resonator coil and the recirculating device may be observed. The resonant frequencies for each of the CW and CCW paths through the optical fiber coil are based on a constructive interference of successively recirculated beams in each optical path. A rotation rate around the axis of the RFOG, i.e. the optical fiber coil's center axis, produces a shift in the resonant frequencies of the CW and CCW paths through the optical fiber coil. The frequency difference associated with tuning the CW beam and CCW beam frequencies to match the optical fiber coil's resonant frequency shifts in the counter-propagating paths due to rotation is indicative the RFOG's rotation rate.
RFOGs are susceptible to bias error due to polarization cross-coupling. Light may be cross-coupled between two polarization states in the fiber coil itself. The second, undesired polarization state may resonate and produce an asymmetry in the resonance line shape of the first, desired polarization state used to measure a rotation. Even though, ignoring other spurious affects, the resonant frequencies of the first and second polarization states are the same for each of the CW and CCW paths, the excitation levels of the resonances for each polarization state may differ between CW and CCW directions, due to different launching conditions. In addition, the detection of light from the resonator in the two polarization states may differ between CW and CCW directions. These conditions result in polarization-induced bias error which can severely limit the accuracy of the RFOG because determination of the resonance centers for each of the resonant frequencies of the CW and CCW paths directly affects the rotational rate measurement. To reduce polarization induced bias error due, RFOG optical fiber coils are conventionally fabricated with highly birefringent optical fiber which supports two polarization states, but is much less susceptible to cross coupling between those modes.
In the RFOG, bias error is caused by the presence of glass material, through which light beams travel, in the optical fiber of the optical fiber coil. The refractive index of the glass material is non-linear with respect to the power of the light beam due to the Kerr effect. If the powers of the CW and CCW light beams differ, so will the refractive index of the corresponding glass material in the CW and CCW paths. This results in a non-reciprocal path length of the RFOG resonator, and an erroneous determined rate of rotation. The glass material gives rise to other spurious effects, such as stimulated Brillouin scattering and a Faraday magneto-optical effect, which also cause bias error.
Problems with solid core fibers are discussed further in U.S. Pat. No. 7,751,055 (hereinafter the “'055 Patent”) which is hereby incorporated by reference herein in its entirety. The '055 Patent discloses solving this problem by using an optical fiber coil made from hollow core optical fiber coil. One hollow core optical fiber disclosed in the '055 Patent is a hollow core bandgap fiber.
Hollow core bandgap fiber, otherwise known as photonic bandgap fiber, is formed by a lattice of glass rods. The lattice of cells forms a region around a hollow core and the core wall of the hollow core; light substantially propagates through the hollow core. The lattice of cells surrounding the hollow core is referred to as the cladding region. Each cell, in the lattice or cladding region, has a glass wall which is referred to here as a strut or a membrane.
Inside the membranes of each cell is free space (not glass), e.g. which may be either vacuum or gas-filled. Gas is defined herein to mean a state of matter between plasma and liquid of:                at least one type of single element (e.g. neon), at least one type of elemental molecule (oxygen), at least one type of compound molecule (e.g. carbon dioxide), or a mixture of one or more of the foregoing (e.g. air).            Gas, as used herein, also means dry gas and gas that is not dry. A dry gas includes no vapor that liquefies at an ambient temperature and pressure. Whereas natural air is an example of a gas that is not dry, dry air is an example of a gas that is dry.
Two adjacent cells share a membrane as a wall; however, where more than two cells meet, a node is formed. The node typically is at the intersection between three membranes.
The walls of some cells along one axis within the cladding may be thicker than others to cause the hollow core bandgap fiber to be highly birefringent, and thus, inhibit optical cross-talk between polarization states within the fiber. Although it eliminates the problematic core of glass material, hollow core bandgap fiber is not without its own problems.
Features within a hollow core bandgap fiber such as the nodes, or imperfections in launching into the hollow core bandgap fiber, cause light propagation in undesirable surface modes; light in surface modes travels in a collection of cells surrounding the core, rather than in the hollow core. Perturbations in the hollow core bandgap fiber later couple the light in surface modes back into the core. Surface modes can create bias error and noise due to back scattering of light in the surface modes and due to surface roughness, as well as multipath effects arising from cross-coupling between light in the surface modes and the core. Therefore there is a need for an RFOG with a hollow core fiber that does not suffer from these bias errors and noise mechanisms.